Vibration level switch systems usually consist of a vibration resonator and an oscillation exiting feedback circuit coupled thereto which during operation of the system excites the vibration resonator to its resonance frequency such that a self-preserving oscillation results.
In prior art, vibration sensors may be known, operating on this principle. For testing the faultless functioning of such a sensor system, instead of the vibration sensor, a band pass filter is connected to the feedback circuit. The generated oscillation frequency, determined by the band pass filter, is stored during an initialization calibration and is compared to reference frequency values obtained during subsequent test measurements.
However, this method for testing the functioning is only suitable for liquid level switch sensors analyzing the frequency. The electronic of the feedback circuit and the subsequent stages are tested for failures by means of this method. A testing of the sensor mechanism is not possible.
Another method for testing the operation of a vibration sensor for level detection of solid bulk material is to set the amplification factor of the feedback circuit to a fixed low value such that with the sensor not covered by bulk material, a signal is output, indicating a full container, or is set to a fixed height value, such that with the sensor covered completely, a signal is output, indicating that the container is empty.
The process is suitable for solid bulk material level switch sensors analyzing the amplitude. However, only very rough changes of the sensor can be detected by this method.
In prior art, another vibration sensor is known, being also excited by means of a feedback circuit to its resonant frequency. For compensating manufacturing tolerances, the switching parameters of the feedback circuit can be changed on the basis of a characteristic element which is assigned to the sensor mechanism. However, with this sensor, no operating tests are possible at all.
Therefore, there would be a need for a method, according to which liquid as well as solid bulk material vibration sensors can be tested for their correct functioning. It would also be meaningful that by means of the operating test, also the oscillation exciting feedback circuit is tested as well as the subsequent signal processing stages, as well as the connection lines to the respective electronics, the electromechanical converter system and finally also the mechanical system of the sensor.
Moreover, it would be meaningful to not only detect failures leading to the complete failure of the vibration sensor but also failures which only result in a change of the measurement properties without leading to complete failure of the sensor system.